1. Field of the Invention
The present invention relates to a method for atomic vapor laser isotope separation of isotopes using two or more photoionization pathways simultaneously. In particular, the isotope .sup.167 Er can be enriched using two three-step photoionization pathways involving the hyperfine structure of .sup.167 Er.
2. Description of Related Art
Erbium is a naturally-occurring element belonging to the rare-earth or lanthanide series, and is made up of six stable isotopes with natural abundances given in parentheses: .sup.162 Er (0.14%), .sup.164 Er (1.61%), .sup.166 Er (33.6%), .sup.167 Er (22.95%), .sup.168 Er (26.8%), and .sup.170 Er (14.9%). Erbium has nuclear and metallurgical applications, and is used as a dopant in glasses and ceramics. Natural erbium is being developed for use in commercial nuclear reactors, with the isotope .sup.167 Er as the active component. The other erbium isotopes dilute and degrade reactor performance, so enrichment of .sup.167 Er is desirable.
Enrichment of one or more isotopes is achieved through separation processes that take advantage of the minute differences in chemical or physical properties between the isotopes. Isotopic separation on a small scale is performed using a mass spectrometer, but large scale methods include gaseous diffusion, distillation, electrolysis, thermal diffusion, centrifuging, and laser methods. Laser isotope separation of an atomic vapor selectively excites energy states in one or more isotopes (without exciting and ionizing other isotopes), and then the selectively ionized isotopes are separated from the neutral atoms in an electromagnetic field.
Laser isotope separation typically exploits isotope shifts, which are displacements in the spectral lines of isotopes of an element. Uranium-235 separation and enrichment for use in commercial power reactors has been demonstrated with plant-scale hardware using an atomic vapor laser isotope separation (AVLIS) process, where the uranium isotope shifts are about 2 GHz per mass unit. In comparison, isotope shifts are small in erbium: about 0.5 GHz per mass unit. In place of isotope shifts, the present method uses the hyperfine structure (hfs) of .sup.167 Er to isolate energy levels for selective photoionization. Isotope enrichment of the even zirconium isotopes by removing .sup.91 Zr has been proposed using the hyperfine structure of this even-odd isotope (P. A. Hackett, H. D. Morrison, O. L. Bourne, B. Simard, and D. M. Rayher, J. Opt. Soc. Am., B5, pp. 2409-2416 (1988)). Their technique has not been applied to or suggested for erbium isotopes.
Designing a scheme for efficient selective ionization of a desired isotope is not readily apparent, but requires investigation of which atomic transitions to use and the particular sources of photon energy required for overall efficiency of the separation process. The discovery of specific energy levels is critical to determine the proper pathways that result in an efficient, commercially viable photoionization scheme. Successful laser isotope separation of erbium isotopes is dependent on finding effective multiple-step excitation pathways that will enrich the desired isotope, .sup.167 Er. The steps must have a number of spectroscopic properties that cannot be predicted, but need to be determined by laboratory experiment.
One requirement of the photoionization steps is that the frequency separations, or isotope shifts, of the desired isotope, .sup.167 Er, and the even erbium isotopes (.sup.162 Er, .sup.164 Er, .sup.166 Er, .sup.168 Er, and .sup.170 Er) are sufficient to allow selective ionization. In addition, the transition absorption cross-sections for all steps must be of appropriate magnitude. The transition frequencies for all steps preferably fall in the range of efficient dye lasers, such as copper vapor laser pumped dye lasers or Nd:YAG laser pumped dye lasers. Applicants have found that fifty to one hundred percent more .sup.167 Er can be photoionized by the simultaneous use of two separate multiple-step photoionization pathways that have a common upper energy level.